Which @angular/* package(s) are the source of the bug?

Don't known / other

Is this a regression?

Yes

Description

When a change is made to HTML/CSS in a client-side rendered project with HMR enabled, the app will be reloaded fully under most scenarios where there is a class declared in addition to a component's class.

Example:

import { Component } from '@angular/core';

class CommentMeInOrOut {}

@Component({
  selector: 'app-class-above',
  template: `
    <p>
        App will reload on template or style change, if CommentMeInOrOut is left in.
        If commented out or removed entirely, HMR works fine. 
        Same in *most* permutations of inline or external file templates/styles.
    </p>
  `,
  styles: `
    p {
      color: green;
    }
  `,
})
export class ClassAbove {}

If the additional class is commented out, then HMR works fine.

EXCEPTION: There seems to be a notable exception for external template files. When an external template is changed, HMR works like intended and does not cause full application reload. I marked those two templates with a star emoji in the templates.

About the reproduction Stackblitz:

• A counter goes up by 1 for every second without an application reload
• I made a bunch of components with different permeations of inline or external file templates/styles.
• Control: One example has no extra class, not even commented out. This works as intended and like the other examples when the extra class is commented out or removed.
• One example has a component with inline template/styles and then another component class which is used in the template of that file's component. If the inline templates/styles of either are changed, the whole app reloads.
• edit: with default change detection and using Zone. Unsure if those would change anything but I have done so many permutations already. Though that said, in the v19.1.4 project I found this in, the zone project using OnPush change detection had the same issues as this repro's equivalent.

Please provide a link to a minimal reproduction of the bug

https://stackblitz.com/edit/stackblitz-starters-sct51wue?file=src%2Fmain.ts

Please provide the exception or error you saw

Please provide the environment you discovered this bug in (run ng version)

Angular CLI: 20.1.2
Node: 20.19.1
Package Manager: npm 10.8.2
OS: linux x64

Angular: 20.1.3
... animations, common, compiler, compiler-cli, core, forms
... platform-browser, router

Package                      Version
------------------------------------------------------
@angular-devkit/architect    0.2001.2
@angular-devkit/core         20.1.2
@angular-devkit/schematics   20.1.2
@angular/build               20.1.2
@angular/cli                 20.1.2
@schematics/angular          20.1.2
rxjs                         7.8.2
typescript                   5.8.2
zone.js                      0.15.0

Anything else?

I initially observed this in a v19.1.4 common / 19.1.5 build project. I did not test all these permutations in that one though.